The present disclosure relates to a bias circuit. A power amplifier that amplifies a radio frequency (RF) signal to be transmitted to a base station is used in a mobile communication terminal such as a cellular phone. The power amplifier includes a transistor that amplifies the RF signal and a bias circuit that controls a bias point of the transistor. As this type of bias circuit, a bias circuit using a current mirror circuit is known as described in, for example, U.S. Patent Application Publication No. 2015/0349715. In addition, as this type of transistor, a heterojunction bipolar transistor is known.
However, a heterojunction bipolar transistor has a large thermal resistance, and thus generates heat as an RF signal output increases, thereby resulting in a lower transconductance. As the transconductance decreases, the gain of the power amplifier decreases. When the RF signal output further increases, the bias point of the heterojunction bipolar transistor increases, and thus the gain of the power amplifier turns to an increase pattern from a decrease pattern. In this manner, the linearity of the power amplifier deteriorates due to changes in the gain of the power amplifier in its low and middle output region. Such linearity deterioration distorts the RF signal, and thus it is not desirable especially for a communication system using a linear modulation scheme.